[SPARK-21175] Reject OpenBlocks when memory shortage on shuffle service.

## What changes were proposed in this pull request?

A shuffle service can serves blocks from multiple apps/tasks. Thus the shuffle service can suffers high memory usage when lots of shuffle-reads happen at the same time. In my cluster, OOM always happens on shuffle service. Analyzing heap dump, memory cost by Netty(ChannelOutboundBufferEntry) can be up to 2~3G. It might make sense to reject "open blocks" request when memory usage is high on shuffle service.

https://github.com/apache/spark/commit/93dd0c518d040155b04e5ab258c5835aec7776fc and https://github.com/apache/spark/commit/85c6ce61930490e2247fb4b0e22dfebbb8b6a1ee tried to alleviate the memory pressure on shuffle service but cannot solve the root cause. This pr proposes to control currency of shuffle read.

## How was this patch tested?
Added unit test.

Author: jinxing <jinxing6042@126.com>

Closes #18388 from jinxing64/SPARK-21175.